This application is based upon and claims benefit of priority of Japanese Patent Application No. 2001-27440 filed on Feb. 2, 2001, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a capacitor-type dynamic sensor, especially to its electrodes and flexible beam structure.
2. Description of Related Art
A capacitor-type dynamic sensor is composed of stationary electrodes and movable electrodes that face the stationary electrodes forming a small air gap therebetween. A dynamic force applied to the sensor, such as acceleration, is detected by changes of a capacitance formed between both electrodes. When an excessive impact is applied to such a conventional sensor, the movable electrodes contact the stationary electrodes, and thereby both electrodes may stick to each other. To suppress such a sticking phenomenon, JP-A11-326365 proposes to form plural projections on the stationary electrodes. Since the movable electrodes contact the stationary electrodes via such projections when an excessive impact is applied to the sensor, a contacting area of both electrodes is decreased and the sticking force therebetween is also decreased.
However, since the projections are uniformly positioned on the stationary electrodes in the dynamic sensor proposed, all the projections may be destroyed at the same time when an excessive impact is imposed. If all the projections are destroyed, the sticking of both electrodes cannot be avoided.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved dynamic sensor, in which the sticking between the movable and stationary electrodes is avoided.
Stationary electrodes and movable electrodes are formed on a semiconductor substrate, so that the movable electrodes face the stationary electrodes with a small air gap therebetween. A capacitance which is variable according to a dynamic force such as acceleration imposed on the dynamic sensor is formed between the stationary electrodes and the movable electrodes. The changes in capacitance are taken out from the sensor as electrical signals, and the dynamic force imposed on the sensor is detected based on the electrical signals.
The movable electrodes tend to stick to the stationary electrodes when both electrodes contact each other due to an excessive impact imposed on the sensor. To avoid or suppress the electrode sticking, plural projections are formed on either the movable electrodes or on the stationary electrodes or on both electrodes. Those projections decrease an area of contact between both electrodes when both electrodes contact each other by an excessive impact. The projections are formed to have various heights so that the higher projections first hit the other electrode thereby to protect lower projections. The lower projections thus protected serve to prevent the electrode sticking which is otherwise caused by the next excessive impact.
The stationary electrodes are supported on the semiconductor substrate at one end thereof, forming a cantilever structure. The movable electrodes are connected at one end thereof to a center rod which is anchored on the semiconductor substrate at its both ends, the movable electrodes also forming a cantilever structure. Due to the cantilever structure of both electrodes, the electrode sticking is most likely to occur at the tip portion and foot portion of the cantilever structure. Therefore, the projections are more densely positioned either at the tip portion or at the foot portion of the cantilever-structured electrodes or at both the tip and foot portions than at a center portion thereof. Further, higher projections are formed at the tip or the foot portions, or at both portions.
It is preferable to form the projections on the stationary electrodes to avoid a dynamic unbalance of the movable electrodes which may occur when some of the projections formed on the movable electrodes are destroyed by an excessive impact. The projections may be formed on both sides of the stationary electrodes facing the movable electrodes to further securely avoid the electrode sticking.
The projections may be made on a flexible beam that swings back and force according to a dynamic force applied thereto. Such projections are made so that the projections restrict an excessive swinging motion of the flexible beam before the movable electrodes contact the stationary electrodes.
The sticking between the moving electrodes and the stationary electrodes is avoided or suppressed by the projections formed according to the present invention, and dynamic sensor can be operated for a long time.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings.